Automatic optical inspection (AOI) systems are commonly used nowadays in order to inspect bare printed circuit boards (PCB) for possible manufacturing defects or flaws, wherein the pcb layout is inspected in accordance with pre-determined design rules. Although the identification of such flaws is amenable to computerized inspection and analysis, a human operator is the final arbiter as to whether the suspected fault is in fact a flaw or not. Furthermore, as is well known, suspected flaws fall into three categories: there are those which, on closer examination, turn out to be false alarms; there are those which are genuine faults but are nevertheless susceptible to minor repair and there are those faults which are irreparable.
A printed circuit board with an irreparable fault is not susceptible to repair and must therefore be discarded. Clearly, no further act/on need be taken in respect of those suspected flaws which, on closer examination, turn out to be false alarms. So far as the second category is concerned, namely those faults which are susceptible to minor repair, it is known to dispatch the printed circuit board to a verification and repair station remote from the AOI system for subsequent repair. Although, obviously, it is possible to effect such repair at the AOI station itself, there are good reasons for not doing so. Specifically, the automatic optical inspection is very fast compared with any subsequent manual repair operation and thus to effect tile repairs at the AOI station itself would create a bottleneck and reduce the throughput and hence efficiency of the AOI station. Furthermore, very often suspected faults turn out, on closer analysis, to be false alarms. It would thus be a waste of time and resources to devote the required effort at the AOI station itself to differentiating between false alarms and genuine but repairable faults.
In order to effect tile required interconnection between the AOI station and tile verification and repair station, the AOI station produces a computer file containing therein a list of coordinates corresponding to the suspected fault locations on the PCB. These coordinates are then scanned sequentially at the verification and repair station so as to be successively imaged by a high magnification camera so that each suspected fault, in turn, may be displayed on a visual display monitor at very high magnification, enabling an operator to determine to which of the three above-mentioned categories the suspected fault belongs and, where relevant, whether it is susceptible to repair.
In such a system a repairable fault is repaired in situ, the operator effecting the repair whilst viewing the high magnification image on the display monitor.
Systems of the kind described are known in both the scientific and patent literature. Thus, for example, the general concepts are disclosed in U.S. Pat. No. 4,758,888 (Lapidot) which discloses a method and system for inspecting workpieces ravelling along a production line, wherein possible flaws are detected at an upstream inspection station without interrupting the progression of workpieces along the production line. Possible flaws are imaged and stored in a memory so as to be associated with the workpiece containing the possible flaw and the stored image is subsequently retrieved to enable verification, by visual inspection, as to whether the possible flaw is valid or not. On this basis, the workpieces are subsequently sorted into satisfactory and defective workpieces, the latter being diverted to a repair or scrape process for subsequent rework where possible.
A similar system is described in the IBM Technical disclosure Bulletin Volume 29 No. 3, August, 1986 pp 1154-1155 in an article entitled "Paperless system for inspection and rework of circuit boards". The system described therein is directed to circuit boards containing actual circuit components rather than to the bare PCB. Furthermore, the inspection record is created from a manual inspection of the board populated with components which also, of course, is a fundamental departure from AOI systems wherein such inspection is, in effect, performed automatically. Nevertheless, the article does disclose the concept whereby the results of the inspection are stored in association with an identification of the board for later recall during any necessary rework of the board.
European Patent Application No. 386 924 (Peles) discloses a work station for orientation of a workpiece, comprising a substantially plane mounting table for mounting the workpiece thereon. Typically, the workpiece is a PCB, predetermined suspected fault locations of which are imaged by a fixed camera by moving the mounting table until the predetermined fault location is aligned with the fixed camera. Such alignment is achieved by a single rotation of the table and a single translation thereof both within the plane of the table. In the opening section of European Patent Application No. 386 924, reference is made to prior art verification and rework stations wherein the desired alignment between the suspected fault location and the fixed camera is achieved by two mutually orthogonal translatory movements within the plane of the table. As is explained in European Patent Application No. 386 924, such verification stations have a large "footprint" requiring a large floor space since, with XY-movement of the table, the net area required for checking a square printed circuit board with a side L is 4L.sup.2. Furthermore, as the table is moved towards the operator in order to align a desired fault location with the illumination system, this inevitably increases the distance of the operator from the fault location. By effecting the desired alignment with a single rotation and a single translation, the required footprint is apparently reduced by up to about 25%. However, no solution is provided to the associated increase in operator distance from the fault location.
FIGS. 5, 6 and 7 of the above-mentioned European patent application show schematically how the system operates. It will be clear particularly from FIGS. 6 and 7 that when the fault location is aligned with the camera, a corner of the table protrudes towards the operator and the edge of the table is consequently no longer parallel to an imaginary line joining the center of the table to the camera. The amount by which the corner of the table thus protrudes is unpredictable and depends on where on the workpiece a particular fault location is situated. As a result, if the table is orientated substantially horizontally, either the operator must sit sufficiently far away from the table so as to accommodate maximal protrusion of the corner of the table as the table rotates about its axis or, alternatively, the operator must be free to move to and fro in order to accommodate the table's rotation.
Correspondingly, if the work table is orientated at an angle to the horizontal as is shown in FIG. 4 of the European patent application, then the work table must be situated sufficiently high relative to the operator's legs in order that maximal protrusion of the corner of the table can never foul the operator's legs.
In either case, these considerations militate somewhat against the requirement to reduce the footprint of the work table and thus demand that a greater area be allocated to the table's movement than is desirable.
Apart from considerations of space, there exist other drawbacks associated with verification and repair stations which have not been addressed in the prior art. It is well known, for example, that as the magnification of an optical system increases, so too does its depth of field diminish. This is highly significant in verification and repair stations of the kind described since the magnification required is so high that any slight distortion in the upper surface of the PCB being imaged can quite easily result in part of that surface being out of focus even when the PCB surface is nominally in focus. This would not happen if the surface of the PCB were exactly flat, since steps could then be taken to ensure precise and uniform focus over the entire imaged area. However, in practice, buckling is difficult to prevent and the problem of imprecise focus therefore exists.
Yet a further consideration is the manner in which the PCB surface is illuminated. In practice, two different sources of illumination are required: one directed from above in order to illuminate the upper surface of the PCB; and one directed from below so as to pass through feed-through holes or vias in the PCB in order that they too can be inspected and, if necessary, repaired.
Directing the illumination from above normally requires that a bright light source is directed over a small area of the PCB in order to illuminate the conductive tracks thereon. Since the conductive tracks themselves are generally highly polished, the light can be reflected thereby and generate flare which reduces the quality of the resulting image.
The manner in which the camera is focused is also an important feature of such verification and repair stations. It is possible to rely on a completely manual focusing system controlled by the operator but this, of course, is relatively time-consuming. Auto-focus systems are commonly used nowadays and their application to verification and repair stations of the kind described is clearly, in itself, merely a matter of design rather than of invention. However, it must be appreciated that whilst the use of auto-focus systems can increase the speed with which the camera is focused, it cannot compensate for buckling or other surface distortions of the PCB which render uniform focusing impossible.